To achieve the best wire dress and ball centering in wire bonding, the tension on the wire must be changed at different phases of the bond cycle. This is especially true as the pin count of the bonded device increases, the pitch between bonded wires decreases, and/or the length of the bonded wire increases. In each of these cases, the need to control the length and shape of the bonded wire is paramount.
Prior art wire tensioners provided a static drag force to the bond wire. This is accomplished, for example, by sandwiching the wire between a felt pad and a metal pad. The felt pad is compressed against the metal pad by an extension spring. Tension on the wire is varied by adjusting a set screw to increase or decrease the compression of the felt pad.
During gold wire bonding a small amount of drag force is required to pull the gold ball on the end of the wire into the capillary before the ball bond is performed. A large force is needed to pull wire through the capillary during the move from ball bond to stitch bond to form an acceptable wire loop. A compromise between the small force needed for ball centering and the large force needed for wire looping is necessary when using a fixed force wire tensioner. Therefore, the force is not optimized for either operation.